1. Field of the Invention
The present invention relates to an optical scanner having a plurality of light sources forming a composite spot and capable of changing the diameter of the composite spot.
As shown in FIG. 1, when a line inclined by 45.degree. is printed in a laser printer, spot diameter D must be larger than pitch d of a scanning line or the inclined line will appear broken. When the intensity distribution of the spot is of an isotropic Gauss type with a central intensity I.sub.o and the spot diameter is defined as a point having intensity I.sub.o /exp(-2), the light intensity Ia at point a is provided by the following formula (1). ##EQU1## When a region having an intensity not less than I.sub.o /exp(-2) is developed, the condition for preventing interruption of a line inclined by 45.degree. is provided by the following inequality (2). EQU I.sub.a .gtoreq.I.sub.o /exp(-2)
From the formula (1) and the inequality (2), the following inequality (3) is formed. EQU D/d.gtoreq.1.22
The dither method is one method for representing the gradation of an image. In the dither method picture elements are decomposed to a matrix and the density of the picture elements changed by changing the coating ratio of this dither matrix. When a dither pattern is printed under the condition of the inequality (3), as shown in FIG. 2, the spot diameter is larger than the area of one dither matrix constituting picture element 71. Therefore, as shown in FIG. 3, the relation between the dither matrix coating ratio and the density of the picture element is not a linear relation. Thus the gradation representation becomes inaccurate in the region in which the dither matrix coating ratio is high. In FIG. 3, the density of the picture element is 1 at its maximum.